1. Field of the Invention
The present invention relates to valves and, more specifically, to a flow regulating valve that regulates the flow of fluid in one direction and allows the generally unregulated flow of fluid in the opposite direction.
2. Description of the Related Art
In vehicles employing hydraulic systems, it is known to employ valves that limit the flow of the hydraulic fluid through a fluid line leading to a hydraulic actuator such as a hydraulic motor or cylinder to a maximum flow rate. For example, in a combine harvester it is known to use an internal combustion engine to power a hydraulic pump. The hydraulic pump provides hydraulic fluid under pressure to a hydraulic circuit. Each of the driven wheels of the combine harvester may include a separate hydraulic motor that is powered by the hydraulic circuit. Each of the motors may be located in a separate loop in communication with the hydraulic circuit. By reversing the direction of flow through the individual loops with the use of a reversing pump, reversing valve or other suitable means, the rotational direction in which the wheel is driven may also be reversed.
In such a loop, it is known to provide two flow regulating valves, one on each side of the hydraulic motor. The flow regulating valves are positioned in the loop such that one of the valves limits the flow of hydraulic fluid through the hydraulic motor, or other hydraulically driven device, to a maximum flow rate in a first direction while the other valve limits the flow to a maximum flow rate in the opposite direction. During operation, while one valve is regulating flow, it is desirable for the other valve in the loop that is not performing a flow regulating function to freely pass the fluid therethrough with a minimal pressure drop and without restricting the flow rate of the fluid.
By providing flow regulating valves in each individual hydraulic motor loop, if one of the driven wheels begins slipping, the flow of hydraulic fluid to the slipping wheel will be limited to the maximum flow rate permitted by the valve. Limiting the flow rate of hydraulic fluid to the slipping wheel prevents excess flow of hydraulic fluid to the slipping wheel from depriving the remaining wheels of a sufficient flow of hydraulic fluid as well as preventing the uncontrolled spinning of the slipping wheel which can result in damage to the turf, cropland and/or tire. FIGS. 1 and 2 illustrate one known example of such a flow-regulating valve.
The valve 10 shown in FIGS. 1 and 2 includes a valve body 12 that receives outlet adapter 14 with an O-ring 15 or other suitable means providing a seal therebetween. Valve body 12 and outlet adapter 14 both include axially extending passages 16 and 18, respectively, for conveying hydraulic fluid. Valve 10 also includes piston 20, baffle member 22 and spring 24 which provide for the regulation of fluid flow through the valve. Valve 10 limits the flow of hydraulic fluid to a predetermined design flow rate when hydraulic fluid is flowing through valve 10 in the direction indicated by flow arrows 26 in FIG. 1. When hydraulic fluid is flowing in this regulated direction, piston 20 will initially be in the position shown in FIG. 2 wherein spring 24 biases piston 20 away from baffle 22 to a point where radial flange 28 engages valve body 12. The flow rate of hydraulic fluid through valve 10 is dependent upon the pressure differential across valve 10. When the flow of hydraulic fluid through calibrated orifice 30 in the direction indicated in FIG. 1 increases, the pressure differential acting on piston 20 will also increase. When the pressure differential and resulting force on the piston 20 exceeds the biasing force of spring 24, piston 20 will be biased towards baffle 22. As piston 20 moves towards baffle 22, the annular orifice 32 defined between piston 20 and baffle member 22 decreases in size thereby restricting the flow of fluid through the valve. By properly selecting the spring and valve dimensions, valve 10 may be used to limit the flow of fluid in the direction indicated in FIG. 1 to a maximum predetermined flow rate.
In FIG. 2, the flow of hydraulic fluid through valve 10 is in the opposite return flow direction as indicated by flow arrows 27. When fluid is flowing in this return direction, there is no fluid flow force to counteract the biasing force of spring 24 and annular orifice 32 maintains a constant size regardless of the flow rate or pressure differential of the hydraulic fluid. Consequently, valve 10 does not positively control the flow rate of the hydraulic fluid through the valve in the return direction and flow is limited by the size of the metering orifice 30. In other words, the valve does not regulate the flow of fluid through the valve when the fluid is flowing in the direction indicated by arrows 27 in FIG. 2 but rather the limited size of metering orifice 30 restricts the flow of fluid through the valve resulting in a pressure drop across the valve and undesirable power losses and heating of the fluid.
Another example of a known flow compensating valve assembly is shown in U.S. Pat. No. 5,320,135. The valve assembly disclosed in this patent may be used with hydraulic cylinders found in hydraulic platform lifts. The compensator valve 1 includes a valve body 10 receiving a sleeve 12 having an upper portion 16 and a lower portion 18. A piston 20 is sliding received within sleeve 12 and, as best seen in FIGS. 3–6, a spring 30 is provided between the bottom end 19 of the lower sleeve portion 18 and the top end wall 21 of piston 20. Piston 20 includes an axial main port 22 and a pair of relief ports 23a and 23b on its side wall periphery portion. In operation, as shown in FIG. 3, when hydraulic fluid is traveling from the pump to the hydraulic cylinder from borehole 44 to bore 41, as shown by the arrows, hydraulic fluid travels around and between the lower sleeve portion 18 and the inner wall portion 54 of the valve body 10 and into sleeve 12 through ports 15a and 15b. As shown, during this condition, piston 20 is forced toward bore hole 41 thereby causing relief ports 23a and 23b to be placed in communication with the relief region 56 of sleeve 12. Thus, flow is provided through side relief ports 23a and 23b as well as through the axial main port 22. When the flow direction is reversed, with fluid flowing from the hydraulic cylinder to the pump, and there is little or no back pressure as depicted in FIG. 4, spring 30 maintains the piston 20 in the fully extended position thereby allowing flow through the ports 23a and 23b. When the flow from the hydraulic cylinder to the motor is increased, as depicted in FIG. 5, piston 20 acts against the spring 30 and travels into sleeve 12 thereby closing off the fluid relief ports 23a and 23b such that flow occurs only through the main axial port 22. As the hydraulic fluid pressure further increases as shown in FIG. 6, the piston exerts yet a greater force against spring 30 traveling further into the sleeve 12 so as to partially block outlet ports 15a and 15b. 
While the valve assembly disclosed in U.S. Pat. No. 5,320,135 may effectively regulate the flow of hydraulic fluid for the hydraulic cylinder of a hydraulic lift, it is not without shortcomings. If such a valve assembly were to be used to limit the flow of hydraulic fluid to a hydraulic motor by placing the valve in a hydraulic motor loop circuit, as shown in FIGS. 4–6 of U.S. Pat. No. 5,320,135, the fluid flow would initially have to overcome the resistance of spring 30 before the valve is moved from the condition shown in FIG. 4 to that shown in FIG. 5. This could result in a relatively rough transition wherein the fluid flow initially increases rapidly while the valve was in the condition of FIG. 4 and then rapidly decreases as the valve is moved to the condition shown in FIG. 5 wherein ports 23a and 23b are closed. The flow rate could then resume its increase until the valve begins to close ports 15a and 15b as depicted in FIG. 6. While this may be acceptable for the operation of a hydraulic lift, such a transition could result in the rough and unacceptable operation of a hydraulic motor driven wheel. This rough transition would likely be particularly evident when the direction of fluid flow to such a hydraulic motor was reversed and fluid flow was initially being increased.
An improved valve assembly is desired which may be used to efficiently regulate the flow of fluid in one direction to a hydraulic device without rapid or rough transitions and, in the other direction, allow unregulated fluid flow with minimal restriction thereby minimizing power losses and heating of the fluid.